1. Technical Field
The present invention relates to a virtual image display apparatus which presents a video formed by an image display element or the like to an observer, and particularly relates to a virtual image display apparatus which is appropriate to a head-mounted display which is mounted on the head of the observer.
2. Related Art
Various technologies have been suggested as an optical system which is embedded in a virtual image display apparatus, such as a head-mounted display (hereinafter, referred to as an HMD), which is mounted on the head of an observer (refer to US Patent Publication No. 2013/0033756, US Patent Publication No. 2005/0219152, and U.S. Pat. No. 5,886,822).
It is desirable that the virtual image display apparatus, such as the HMD, reduce an apparatus weight for improving mounting comfort. Since it is advantageous to bend an optical path by using a reflecting surface in order to make the optical system small and reduce the weight, various optical systems which use a prism have been developed.
For example, in US Patent Publication No. 2013/0033756, an optical system, which is configured to have a prism in a flat board shape or in a prismatic column shape and one reflecting surface, is illustrated. The optical system has an extremely simple configuration and is advantageous in reducing the weight. However, for example, when a chromatic aberration of magnification remains and an image having a wide angle of view or a high resolution is formed, there is a case where an optical performance is not sufficient.
In US Patent Publication No. 2005/0219152, an optical system, which is configured to have a prism in a flat board shape or in a prismatic column shape and a plurality of lenses, is illustrated. However, the exemplified optical system has a complicated configuration with a great number of lenses. In addition, a light flux (luminous flux) emitted from a video element is inclined with respect to a normal line of a panel, and there is a possibility that a nonuniformity of the video or a deterioration of efficiency occurs.
In U.S. Pat. No. 5,886,822, an optical system, which is configured to have a prism in a flat board shape or in a prismatic column shape and a plurality of lenses, is illustrated. In the optical system, when a lens is disposed between the prism and the observer, and makes it possible to observe the external light, the lens has an influence on a see-through manner. By disposing a concave lens on an object side of the prism, it is possible to make visibility close to zero. However, in this case, since a magnification differs from that in a state of naked eyes, there is a possibility that an uncomfortable feeling is generated when the observer is a person who usually does not wear glasses, and quality of the see-through manner deteriorates.